A capillary number for physical displacement of TCE-DNAPL trapped in a rough-walled fracture

Abstract

A heterogeneous nature of rough-walled rock fractures complicates a recovery of DNAPL trapped in fractured rock aquifers, which forms a persistent plume within the fractured rocks and present a long-term source of contamination. Experimental studies were conducted to suggest a criterion for the physical displacement of DNAPL from rough-walled fractures in the context of capillary number (Nca), a dimensionless number representing the ratio of the viscous to capillary forces. A series of experiments using water, surfactant solution and dense fluid were conducted in the range of capillary number from 2.29 × 10−4 to 3.41 × 10−2. The experimental results suggested that a higher capillary number than 1 × 10−2 be required for a near complete DNAPL removal (more than 95% of initial saturation) from rough-walled fractures. For capillary number on the order of 10−3, DNAPL can be mobilized or recovered to some degree. For the capillary number below 1 × 10−3, any kind of remedial fluids would have no effect on physical displacement. It is suggested that remedial fluid utilizing physical displacement be formulated to comply with Nca > 1 × 10−2 to maximize a complete physical removal and the fluid utilizing chemical solubilization or reaction do with Nca <1×10−3 to minimize unwanted migration.